The invention relates to steerable rotary drilling systems and provides, in particular, apparatus and methods for determining the instantaneous rotational orientation of a rotating drill bit, (the roll angle), in such a system.
When drilling or coring holes in sub-surface formations, it is sometimes desirable to be able to vary and control the direction of drilling, for example to direct the borehole towards a desired target, or to control the direction horizontally within the payzone once the target has been reached. It may also be desirable to correct for deviations from the desired direction when drilling a straight hole, or to control the direction of the hole to avoid obstacles.
"Rotary drilling" is defined as a system in which a downhole assembly, including the drill bit, is connected to a drill string which is rotatably driven from the drilling platform. The established methods of directional control during rotary drilling involve variations in bit weight, r.p.m. and stabilisation. However, the directional control which can be exercised by these methods is limited and conflicts with optimising bit performance. Hitherto, therefore, fully controllable directional drilling has normally required the drill bit to be rotated by a downhole motor, either a turbine or PDM (positive displacement motor). The drill bit may then, for example, be coupled to the motor by a double tilt unit whereby the central axis of the drill bit is inclined to the axis of the motor. During normal drilling the effect of this inclination is nullified by continual rotation of the drill string, and hence the motor casing, as the bit is rotated by the motor. When variation of the direction of drilling is required, the rotation of the drill string is stopped with the bit tilted in the required direction. Continued rotation of the drill bit by the motor then causes the bit to drill in that direction.
The instantaneous rotational orientation of the motor casing is sensed by survey instruments carried adjacent the motor and the required rotational orientation of the motor casing for drilling in the appropriate direction is set by rotational positioning of the drill string, from the drilling platform, in response to the information received in signals from the downhole survey instruments. A similar effect to the use of a double tilt unit may be achieved by the use of a "bent" motor, a "bent" sub-assembly above or below the motor, or an offset stabiliser on the outside of the motor casing. In each case the effect is nullified during normal drilling by continual rotation of the drill string, such rotation being stopped when deviation of the drilling direction is required.
Although such arrangements allow accurately controlled directional drilling to be achieved, using a downhole motor to drive the drill bit, there are reasons why rotary drilling is to be preferred.
Thus, rotary drilling is generally less costly than drilling with a downhole motor. Not only are the motor units themselves costly, and require periodic replacement or refurbishment, but the higher torque at lower rotational speeds permitted by rotary drilling provide improved bit performance and hence lower drilling cost per foot.
Also, in steered motor drilling considerable difficulty may be experienced in accurately positioning the motor in the required rotational orientation, due to stick/slip rotation of the drill string in the borehole as attempts are made to orientate the motor by rotation of the drill string from the surface. Also, rotational orientation of the motor is affected by the wind-up in the drill string, which will vary according to the reactive torque from the motor and the angular compliance of the drill string.
Accordingly, some attention has been given to arrangements for achieving a fully steerable rotary drilling system.
For example, Patent Specification No. WE090/05235 describes a steerable rotary drilling system in which the drill bit is coupled to the lower end of the drill string through a universal joint which allows the bit to pivot relative to the string axis. The bit is contra-nutated in an orbit of fixed radius and at a rate equal to the drill string rotation but in the opposite direction. This speed-controlled and phase-controlled bit nutation keeps the bit heading off-axis in a fixed direction.
British Patent Specification No. 2246151 describes an alternative form of steerable rotary drilling system in which an asymmetrical drill bit is coupled to a mud hammer. The direction of the borehole is selected by selecting a particular phase relation between rotation of the drill bit and the periodic operation of the mud hammer.
U.S. Reissue Pat. No. Re 29526 describes a steerable rotary drilling system in which a pendulum is mounted in the drill pipe close to the bit to assume a vertical position in the azimuthal plane of the drill pipe. When the position of the pendulum is such that the inclination of the drill pipe is not a preselected amount or the azimuthal direction of the pipe is not the preselected direction, a lateral force is imposed on the drill bit urging it to drill in a direction that will return the drill pipe to the preselected inclination or azimuthal direction. The pendulum and its associated apparatus are roll stabilised, that is to say they are rotated in the direction opposite the direction that the drill pipe is rotated and at the same speed, so that the pendulum is substantially non-rotative relative to the earth.
In all of the above-described arrangements it is necessary, in order to achieve the required control, to be able to determine continuously the instantaneous rotational orientation of the rotating drill bit (or in practice a drill collar or other rotatable part associated therewith) since the rotational orientation of the bit at any instant is an essential input parameter for the control system. The instantaneous rotational orientation of the drill bit may be derived from downhole instrumentation, but problems arise in deriving signals which indicate the instantaneous rotational position of the bit with the necessary accuracy, since such signals are liable to be corrupted by high frequency vibrations resulting from the rotation of the drill string.
In the case where the drill bit is driven by a downhole motor, as explained above, rotation of the drill string is stopped when deviation of the drilling direction is required. The downhole instrumentation is therefore non-rotating when measuring the rotational orientation of the drill collar. Accordingly, the signals from the downhole instruments are unvarying (or varying only slowly) and any corruption of the signals by high frequency vibration may therefore be readily filtered out. Such filtering may be effected by processing the signals electronically or by employing instruments which are inherently unresponsive to high frequency vibration. The rotational orientation of the drill collar may therefore be readily computed using signals from sensors in the form of triads of mutually orthogonal linear accelerometers or magnetometers.
In many types of steerable rotary drilling system, however, measurements of the instantaneous rotational orientation of the drill collar must be taken continuously while the drill collar is rotating, and as a result of this there ma be substantial difficulty in obtaining from the sensors signals which are uncorrupted by high frequency vibration or in filtering out such corruption.
With the drill collar rotating, the principle choice is between having the instrument package, including the sensors, fixed to the drill collar and rotating with it, (a so-called "strapped-down" system) or having the instrument package remain essentially stationary as the drill collar rotates around it (a so-called "roll stabilised" system).